1. Field of the Invention
This invention relates to an apparatus for projecting a reticle (a mask) having a predetermined pattern onto the surface of a wafer to which photoresist is applied, by a projection lens, to thereby print the predetermined pattern on the surface of the wafer, and in particular to improvements in a so-called alignment device for effecting alignment of the reticle and the wafer in such an apparatus.
2. Related Background Art
It is usual to use, as a projection lens for projecting and exposing a pattern on a reticle onto a wafer, a lens which is telecentric on the wafer side so that no error of projection magnification occurs even for some focus deviation on the wafer surface. In such a projection exposure apparatus, one of relative alignment systems for the reticle and the wafer is a so-called TTL (through the lens) alignment system for aligning the reticle and the wafer through a projection objective lens.
In such TTL alignment system also, it is desirable that alignment light be telecentric on the wafer side with respect to the projection objective lens. This is for preventing any focus error on the wafer side from affecting the alignment accuracy during alignment. That is, the reason is that even if there is any focus error, the alignment light causes no positional deviation thereof on the wafer.
Generally, projection objective lenses are telecentric on the wafer side, but there are some projection objective lenses which are telecentric on the reticle side as well or which are non-telecentric on one of the wafer side and the reticle side. (These will hereinafter be referred to as the both-side telecentric projection objective lens and the one-side telecentric projection objective lens.)
In the one-side telecentric projection objective lens, it is necessary to incline the principal ray by a desired angle with respect to the optic axis in the other portions than the optic axis in order to maintain telecentricity on the wafer side, and this angle differs depending on the position on the reticle. Accordingly, where the relative position of the reticle and the wafer is detected through the one-side telecentric projection objective lens, the reticle side principal ray has an inclination with respect to the reticle surface at the other points than the optic axis, and this inclination differs specifically depending on the distance from the optic axis and therefore, it is liable to be affected by the focus error on the wafer side. That is, when the principal ray of the alignment light does not form a desired angle with respect to the reticle, the principal ray of the alignment light is inclined with respect to the optic axis of the projection lens on the wafer side, and the position of the mark of the wafer aligned with the mark of the reticle changes in conformity with the presence or absence of the focus error. Also, the images position of an optical signal from the wafer moves on the light-receiving surface of optical signal detecting means, and this has led to the tendency that alignment becomes difficult to accomplish.
Now, even in the both-side telecentric projection objective lens, it is difficult due to the aberration of the pupil of the lens to keep perfect telecentricity. Accordingly, even if as shown in FIG. 1 of the accompanying drawings, a light ray 100 is parallel to the optic axis 400 of the both-side telecentric projection lens 1 on the reticle R side of the projection lens, the light ray 100 will be inclined with respect to the optic axis 400 of the projection lens on the wafer side of the projection lens under the influence of the aberration. Dots-and-dash line 300 is a line parallel to the optic axis 400 of the projection lens 1. When the wafer is positioned on a surface 500 conjugate with the reticle R with respect to the projection lens, the light ray 200 reflected by the wafer enters the same position as the light ray 100 relative to the reticle R, while the light ray 200 follows an optical path different from the light ray 100 and thus, there arises a problem similar to that peculiar to the aforedescribed one-side telecentric projection objective lens. The angle formed by and between the light ray 100 on the wafer side and the line 300 is varied by the amount of relative derivation between the light ray 100 and the optic axis 400. Further, when the wafer is positioned on a surface 500' which is not conjugate with the reticle R with respect to the projection lens 1 due to any focus error, the light ray 200 reflected by the wafer enters a position different from the light ray 100 relative to the reticle R and therefore, it is affected by the focus error as in the aforedescribed one-side telecentric projection objective lens.